Perform a merge between two branches incrementally. If conflicts are encountered, figure out exactly which pairs of commits conflict, and present the user with one pairwise conflict at a time for resolution.
git-imerge has two primary design goals:
- Reduce the pain of resolving merge conflicts to its unavoidable minimum, by finding and presenting the smallest possible conflicts: those between the changes introduced by one commit from each branch.
- Allow a merge to be saved, tested, interrupted, published, and collaborated on while it is in progress.
I think that it is easiest to understand the concept of incremental
merging visually, and therefore I recommend the video of my
git-imerge presentation from the GitMerge 2013 conference (20 min)
as a good place to start. The full slides for that talk are available
in this repository under doc/presentations/GitMerge-2013. At the
same conference, I was interviewed about git-imerge by Thomas
Ferris Nicolaisen for his GitMinutes Podcast #12.
To learn how to use the git-imerge tool itself, I suggest the blog
article git-imerge: A Practical Introduction and also typing
git-imerge --help and git-imerge SUBCOMMAND --help. If you
want more information, the theory and benefits of incremental merging
are described in minute detail in a series of blog articles [1], as
are the benefits of retaining history when doing a rebase [2].
Multiple incremental merges can be in progress at the same time. Each
incremental merge has a name, and its progress is recorded in the Git
repository as references under refs/imerge/NAME. The current
state of an incremental merge can be visualized using the diagram
command.
An incremental merge can be interrupted and resumed arbitrarily, or even pushed to a server to allow somebody else to work on it.
git-imerge comes with a Bash completion script. It can be installed
by copying git-imerge.bashcomplete to the place where usually completion
scripts are installed on your system, e.g. /etc/bash_completion.d/.
git-imerge requires:
A Python interpreter; either
- Python 2.x, version 2.6 or later. If you are using Python
2.6.x, then you have to install the
argparsemodule yourself, as it was only added to the standard library in Python 2.7. - Python 3.x, version 3.3 or later.
The script tries to use a Python interpreter called
pythonin yourPATH. If your Python interpreter has a different name or is not in yourPATH, please adjust the first line of the script accordingly.- Python 2.x, version 2.6 or later. If you are using Python
2.6.x, then you have to install the
A recent version of Git.
Bash completion requires Git's completion being available.
To start a merge or rebase operation using git-imerge, you use
commands that are similar to the corresponding git commands:
git-imerge command |
git analogue |
Effect |
|---|---|---|
git-imerge merge BRANCH |
git merge BRANCH |
Merge BRANCH into the current branch. |
git-imerge rebase BRANCH |
git rebase BRANCH |
Rebase the current branch on top of BRANCH |
git-imerge revert COMMIT |
git revert COMMIT |
Add a new commit that undoes the effect of COMMIT |
git-imerge revert COMMIT1..COMMIT2 |
git revert COMMIT1..COMMIT2 |
Add new commits that undo the effects of COMMIT1..COMMIT2 |
git-imerge drop COMMIT |
git rebase --onto COMMIT^ COMMIT |
Entirely delete commit COMMIT from the history of the
current branch |
git-imerge drop COMMIT1..COMMIT2 |
git rebase --onto COMMIT1 COMMIT2 |
Entirely delete commits COMMIT1..COMMIT2 from the history
of the current branch |
(git-imerge drop is also analogous to running git rebase
--interactive, then deleting the specified commit(s) from the
history.)
A few more options are available if you start the incremental merge
using git imerge start:
git-imerge start --name=NAME --goal=GOAL [--first-parent] BRANCH
where
NAME- is the name for this merge (and also the default name of the branch to which the results will be saved).
GOAL- describes how you want to simplify the results (see next section).
After the incremental merge is started, you will be presented with any
conflicts that have to be resolved. The basic procedure is similar
to performing an incremental merge using git:
while not done:
<fix the conflict that is presented to you>
<"git add" the files that you changed>
git-imerge continue
When you have resolved all of the conflicts, you finish the incremental merge by typing:
git-imerge finish
That should be enough to get you going. All of these subcommands have additional options; to learn about them type:
git-imerge --help git-imerge SUBCMD --help
When the incremental merge is finished, you can simplify its results
in various ways before recording it in your project's permanent
history by using either the finish or simplify command. The
"goal" of the incremental merge can be one of the following:
mergekeep only a simple merge of the second branch into the first branch, discarding all intermediate merges. The end result is similar to what you would get from
git checkout BRANCH1 git merge BRANCH2
rebasekeep the versions of the commits from the second branch rebased onto the first branch. The end result is similar to what you would get from
git checkout BRANCH2 git rebase BRANCH1
rebase-with-historylike
rebase, except that it retains the old versions of the rebased commits in the history. It is equivalent to merging the commits fromBRANCH2intoBRANCH1, one commit at a time. In other words, it transforms this:o---o---o---o BRANCH1 \ A---B---C---D BRANCH2into this:
o---o---o---o---A'--B'--C'--D' NEW_BRANCH \ / / / / --------A---B---C---DIt is safe to rebase an already-published branch using this approach. See [2] for more information.
full- don't simplify the incremental merge at all: do all of the intermediate merges and retain them all in the permanent history.
When git-imerge needs to ask the user to do a merge manually, it
creates a temporary branch refs/heads/imerge/NAME to hold the
result. If you want to suspend an incremental merge to do something
else before continuing, all you need to do is abort any pending merge
using git merge --abort and switch to your other branch. When you
are ready to resume the incremental merge, just type git imerge
continue.
If you need to completely abort an in-progress incremental merge,
first remove the temporary branches git-imerge creates using
git-imerge remove, then checkout the branch you were in before you
started the incremental merge with git checkout ORIGINAL_BRANCH.
git-imerge records all of the intermediate state about an
incremental merge in the Git object database as a bunch of references
under refs/imerge/NAME, where NAME is the name of the imerge:
refs/imerge/NAME/statepoints to a blob that describes the current state of the imerge in JSON format; for example,- The tips of the two branches that are being merged
- The current "blocker" merges (merges that the user will have to do by hand), if any
- The simplification goal
- The name of the branch to which the result will be written.
refs/imerge/NAME/manual/I-Jandrefs/imerge/NAME/auto/I-Jrefer to the manual and automatic merge commits, respectively, that have been done so far as part of the incremental merge.IandJare integers indicating the location(I,J)of the merge in the incremental merge diagram.
It might sometimes be convenient to transfer an in-progress incremental merge from one Git repository to another. For example, you might want to make a backup of the current state, or continue an imerge at home that you started at work, or ask a colleague to do a particular pairwise merge for you. Since all of the imerge state is stored in the Git object database, this can be done by pushing/fetching the references named in the previous section. For example,
git push --prune origin +refs/imerge/NAME/*:refs/imerge/NAME/*
or
git fetch --prune origin +refs/imerge/NAME/*:refs/imerge/NAME/*
Please note that these commands overwrite any state that already existed in the destination repository. There is currently no support for combining the work done by two people in parallel on an incremental merge, so for now you'll just have to take turns.
git rerere is a nice tool that records how you resolve merge
conflicts, and if it sees the same conflict again it tries to
automatically reuse the same resolution.
Since git-imerge attempts so many similar test merges, it is easy
to imagine rerere getting confused. Moreover, git-imerge
relies on a merge resolving (or not resolving) consistently if it is
carried out more than once. Having rerere store extra information
behind the scenes could therefore confuse git-imerge.
Indeed, in testing it appeared that during incremental merges, the
interaction of git-imerge with rerere was sometimes causing
merge conflicts to be resolved incorrectly. Therefore, git-imerge
explicitly turns rerere off temporarily whenever it invokes git.
When git imerge continue or git imerge record finds a resolved
merge in the working tree, it commits that merge then incorporates it
into the incremental merge. Usually it just uses Git's autogenerated
commit message for such commits. If you want to be prompted to edit
such commit messages, you can either specify --edit on the command
line or change the default in your configuration:
git config --global imerge.editmergemessages true
git-imerge is released as open-source software under the GNU
General Public License (GPL), version 2 or later. See file COPYING
for more information.
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